A treatment system is, for instance, a high-dose rate (HDR) brachytherapy system. A HDR brachytherapy system comprises a brachytherapy catheter to be introduced into a target region, wherein within the brachytherapy catheter a radioactive radiation source is moved to different dwell positions at which the radioactive radiation source is located for respective dwell times. The target region is treated by radioactive radiation emitted by the radioactive radiation source at the different dwell positions for the respective dwell times.
Before performing the HDR brachytherapy treatment, a treatment plan is generated, wherein the treatment plan includes, for instance, a planned position of the brachytherapy catheter within the subject. While a physician introduces the brachytherapy catheter into the subject, the position of the brachytherapy catheter may be tracked by using an electromagnetic tracking unit and the tracked position of the brachytherapy catheter may be shown together with the planned position of the brachytherapy catheter on a display for guiding the physician. After the physician has introduced the brachytherapy catheter into the subject, there might be a deviation between the current position of the brachytherapy catheter and the planned position of the brachytherapy catheter, i.e. the planned position and the current position may not perfectly match to each other. It is then difficult for the physician to assess whether this deviation and hence a treatment performed by using this positioning of the brachytherapy catheter is good enough, i.e. acceptable, or whether a repositioning is required.